Many devices, such as protective passive restraints or "air bags" used in motor vehicles, escape slide chutes, life rafts, and the like, are normally stored in a deflated state and are inflated with gas at the time of need. Such devices are generally stored and used in close proximately to humans and, therefore must be designed with a high safety factor which is effective at all times.
Inflation is generally accomplished by means of a gas, such as air, nitrogen, carbon dioxide, helium, and the like which is stored under pressure and further pressurized and supplemented at the time of use by the addition of high temperature combustion gas products produced by the burning of a gas-generating composition. In some cases, the inflation gases are solely produced by gas-generating compositions.
It is obviously very important that the gas-generating composition be capable of safe and reliable storage without decomposition or ignition at normal temperatures which are likely to be encountered in a motor vehicle or other storage environment as, for example, up to temperatures as high as about 110.degree. C. It is also important that substantially all of the combustion products generated during use be non-toxic, non-corrosive, and non-flammable, particularly where the device is used in a closed environment such as a passenger compartment of a motor vehicle.
Igniters for igniting gas generating compositions in inflators for protective passive restraints or "air bags" used in motor vehicles are known. Such igniters are themselves ignited by initiators, e.g., electric squibs, which are activated upon a sensed impact of the motor vehicle.
U.S. Pat. Nos. 4,561,675 to Adams et al and 4,858,951 to Lenzen disclose ignition devices for protective passive restraints or "air bags" in which the igniter and inflator are each contained in aluminum housings. As discussed in each of these patents, the use of aluminum has become prevalent in order to reduce weight. As further discussed in each of these patents, the use of aluminum housings has a disadvantage in that when exposed to high temperatures, such as those which might be encountered in a fire, the mechanical strength of the aluminum depreciates. In such instances when the auto-ignition temperature of the igniter is reached, the aluminum housings can rupture or burst, sending pieces and fragments flying in all directions.
In order to prevent serious damage which may result when igniters and/or gas generating compositions auto-ignite in heated aluminum housings, both U.S. Pat. Nos. 4,561,675 to Adams et al and 4,858,951 to Lenzen provide igniters which have a low auto-ignition temperature. Adams et al rely upon "intimate" thermal contact of the ignition material with the wall of the housing shell. Lenzen utilizes a homogeneous mixture of a booster material and an auto-ignition material which is a smokeless powder that ignites at a temperature in the range of 300.degree. F. to 400.degree. F.
Although the prior art has recognized and addressed the problem of dangerously high auto-ignition temperatures of igniters and/or gas generating compositions, presently known compositions which lower the auto-ignition temperatures disadvantageously suffer extensive weight loss over required storage temperatures, indicting thermal instability which can adversely affect the required performance of these materials.